The following background discussion is not an admission that anything discussed below is citable as prior art or common general knowledge.
In water (including wastewater) treatment systems, a flow of water may need to be provided and controlled past a wall separating two or more open structures containing water. For example, flow may be required between primary and secondary treatment tanks in a water treatment plant or between biological process and separation tanks in a wastewater treatment plant. In a membrane bioreactor (MBR), a flow may be required from a feed channel to a membrane tank or from a membrane tank to a return channel, or both. The word “tank” will be used in this specification for brevity to refer to open structures containing water generally, including tanks and channels as described above and related structures.
In large-scale plants, for example municipal water supply or wastewater treatment plants, the flow rate of water past a dividing wall between adjacent tanks can be very large. To provide and control flow past a dividing wall, fully or partially submerged valves are typically used. For example, knife-gate or sluice-gate valves may be used. The valves add to the cost of a plant, require periodic maintenance and sometimes leak. The valves also create various other significant costs and operational disadvantages that may be less apparent.
For example, the cost of a dividing wall increases if a valve will be placed in it. A dividing wall is often made of concrete on site and needs additional formwork to provide an opening that can be fitted with a valve. The force of water exerted on the valve includes dynamic forces and is concentrated on the wall around the opening. Extra reinforcement is required in the wall to provide the required wall strength, accounting for weakening created by the opening and a need to avoid deflections of the wall beyond the sealing limits of the valve. In a large plant, the opening in a concrete wall required for a conventional valve may be as large as about 7′×7′ and the valve may need to stop water flowing at a velocity of about 1.5 ft/s. The total force on the valve can exceed 16,000 lbs, and this load has to be transferred to the walls around the opening, and the wall must be reinforced to withstand this load. The valve itself also requires structural reinforcing to keep deflections within allowable limits.
Conventional submerged valves through a dividing wall also require a way to service the valve. For example, servicing may require a shutdown and draining of both tanks. This in turn requires a way to operate the plant with the affected tanks temporarily drained. Alternatively, a secondary means for isolating the flow has to be provided. The secondary isolation means may be a second valve or “stop logs” installed on one side of a wall to block the flow of water so that a tank on the other side of the wall can be drained.
In some plants, particularly drinking water plants, a means may also be required to prevent cross-contamination between adjacent tanks through leaks within the design tolerance of the valve. This is particularly of concern when cleaning or disinfecting solutions are present in one tank and it is unacceptable to have the chemicals diffuse into the other tank. In these cases, a “block & bleed valve arrangement” is provided. This involves providing two valves so that a space between the valves can be vented and drained.
In some plants, baffling is also required to reduce the impact of water flowing through the valve on equipment in the downstream tank. For example, in a membrane bioreactor the membrane tanks contain membrane cassettes that can be damaged by strong horizontal flows. With conventional submerged valves, a vertical baffle is required so the incoming water does not hit with full force on the first downstream membrane cassette in the tank.
Siphon valves have been used as an alternative to submerged valves in membrane systems for drinking water and MBRs used for wastewater treatment. The siphon valve uses a closed conduit forming a path generally in the shape of an inverted “U” to transfer liquid over a dividing wall between two tanks. Flow is started by sucking liquid into the conduit and stopped by venting the conduit. However, these siphon valves do not provide rate of flow control and they have not been made with flow capacities adequate for very large plants. Although siphon valves inherently provide some advantages over submerged valves, such as preventing cross-contamination, submerged valves are still the dominant form of flow control device in water treatment systems.